The carburettor on most homeowner-type string trimmers comes equipped with adjustable screws. This allows you to manually regulate the amount of air and gas mixing inside the mixing chamber of the carburettor. While the carburettor comes set from the factory, after using about five to ten tanks worth of fuel you will probably need to reset the carburettor to find that optimal mixing point. Adjusting the carburettor will usually also solve most minor performance and engine problems, such as a sluggish engine or poor acceleration.
Undo the screws holding the carburettor/air filter cover in place over the carburettor and air filter, lifting the cover off. Pop out the foam air filter. Wash the filter in warm, soapy water. Let the filter stand overnight to dry thoroughly before reusing.
Undo the bolts holding the front cover in place over the muffler. Undo the screw holding the spark arrester screen in place over the side of the muffler. Wash the screen in the warm, soapy water. Let it dry thoroughly before reusing.
Empty all of your string trimmer's old fuel into an approved fuel container. Mix new 2-cycle fuel, using regular unleaded gasoline and 2-cycle mixing oil at a ratio of at least 40:1. Note the performance at this mix level and, if performance is still sluggish after adjusting the carburettor, richen the mixture to 50:1.
Start the string trimmer's engine. Squeeze the throttle lightly to allow the engine to warm up for a few minutes. Don't try to adjust the carburettor on a cold engine. Locate the carburettor adjusting screws, which generally can be found near the air filter.
Let go of the throttle trigger and insert the tip of the small screwdriver into the low-speed screw, which is usually marked with an "L." Turn the screw in clockwise until it is fully seated. Note the higher pitched noise of the engine.
Turn the screw out counterclockwise, noting the lower pitched noise of the engine. Fine-tune back clockwise slowly using 1/8 increments to find the optimal engine running sound, which will be a mix of low and higher noises, but will sound clean and smooth.
Insert the tip of the screwdriver into the idle-speed screw, which is usually marked with an "I" or "LA." Turn the screw clockwise until the trimmer's head begins rotating without depressing the throttle. Adjust the screw back counterclockwise until the head stops rotating.
Readjust the low-speed screw after adjusting the idle screw. Use small turns to locate the best engine performance. Continue to monitor the engine noises while trimming and make adjustments as needed.
Don't adjust your high-speed setting without professional help as you can seriously damage your engine.